The present invention relates to methods and apparatus for use in wireless (mobile) telecommunications systems. In particular, embodiments of the invention relate to interference considerations in telecommunications systems.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems.
For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and the coverage area of these networks, i.e. locations where access to the networks is possible, is expected to increase rapidly.
The anticipated widespread deployment of third and fourth generation networks has led to the parallel development of a class of devices and applications which, rather than taking advantage of the high data rates available, instead take advantage of the robust radio interface and increasing ubiquity of the coverage area. Examples include so-called machine type communication (MTC) applications, which are typified by semi-autonomous or autonomous wireless communication devices (i.e. MTC devices) communicating small amounts of data on a relatively infrequent basis. Examples include so-called smart meters which, for example, are located in a customer's house and periodically transmit information back to a central MTC server relating to the customers consumption of a utility such as gas, water, electricity and so on. Further information on characteristics of MTC-type devices can be found, for example, in the corresponding standards, such as ETSI TS 122 368 V10.530 (2011 July)/3GPP TS 22.368 version 10.5.0 Release 10) [1]. Some typical characteristics of MTC type terminal devices/MTC type data might include, for example, characteristics such as low mobility, high delay tolerance, small data transmissions, infrequent transmission and group-based features, policing and addressing.
Whilst it can be convenient for a terminal such as an MTC type terminal to take advantage of the wide coverage area provided by a third or fourth generation mobile telecommunication network there are at present disadvantages. Unlike a conventional third or fourth generation terminal device such as a smartphone, an MTC-type terminal is preferably relatively simple and inexpensive and able to operate on relatively low resources (e.g. low power consumption). The type of functions performed by the MTC-type terminal (e.g. collecting and reporting back data) do not require particularly complex processing to perform, and furthermore are typically not time-critical. However, third and fourth generation mobile telecommunication networks typically employ advanced data modulation techniques on the radio interface which can be power hungry and require more complex and expensive radio transceivers to implement. It is usually justified to include such complex transceivers in a smartphone as a smartphone will typically require a powerful processor to perform typical smartphone type functions. However, as indicated above, there is now a desire to use relatively inexpensive and less complex devices able to operate with low resource usage to communicate using LTE type networks. To this end, so-called “virtual carriers” have been proposed and some characteristics of these are discussed further below.
The increasingly widespread deployment of wireless telecommunications systems can give rise to more likelihood of interference between neighbouring cells. This is particularly the case for LTE based systems which generally adopt a unit frequency reuse approach in which neighbouring cells employ the same radio frequencies. This means a terminal device at a boundary between two communication cells can be receiving comparable signal levels from different base stations using the same frequency resources, thereby leading to potentially significant interference. There have been proposals to address such intercell interference in LTE-type networks using what are generally referred to as Intercell Interference Coordination (ICIC) techniques.
One ICIC technique is the so-called soft frequency reuse approach. In accordance with this technique a base station allocates resources to different terminal devices using different frequencies depending on the respective terminal devices' geographic locations. In particular, neighbouring base stations coordinate so that at geographic locations in the vicinity of boundaries between two communication cells (coverage areas) associated with two base stations, one of the base stations communicates with its connected terminal devices in one frequency band, whilst the other base station communicates with its connected terminal devices in a different frequency band. Thus, in accordance with this approach, a terminal device near a boundary between two cells and connected to a first base station is less prone to interference from a neighbouring base station because the neighbouring base station will be using a different frequency band to serve its connected terminal devices in the vicinity of the cell boundary at that location. Each base station may communicate with terminal devices away from cell boundaries (e.g. at cell centre) using all frequencies, with such transmissions typically being made at a lower power than for transmissions associated with terminal devices in the vicinity of the cell edge.
Another ICIC technique has been proposed for telecommunications networks including a macro base station serving an area that includes one or more femto- or pico-cells (served by respective femto- or pico-base stations). This technique uses so-called Almost Blank Subframes. In accordance with this approach, the base station will select subframes in which it will make almost no transmissions (for example retaining only cell reference signals) and will communicate the timings of these “almost blank subframes” to network elements supporting communications in the femto- or pico-cells. Communications within the femto-/pico-cells can then be coordinated to occur during the subframes in which the macro base station is transmitting an almost blank subframe, thereby reducing the potential for the macro base station to interfere with communications in the femto-/pico-cells.
More details on ICIC techniques, such as those described above, can be found in a paper published by Nomor Research GmbH—“Heterogeneous LTE Networks and Intercell Interference Coordination” by Pauli et al. and also from an article published by ZTE Corporation—“Enhanced ICIC for LTE-A HetNet” by Xiong. Details regarding the protocols for coordinating communications between base stations, for example on the X2 interface of an LTE network, can be found in the relevant standards, for example in ETSI TS 136 420 V11.0.0 (2012 October)/3GPP TS 36.420 version 11.0.0 (Release 11) [15] and 3GPP TS 36.423 version 11.2.0 (Release 11) [16].
The present inventors have recognised how the proposed introduction of virtual carriers in wireless telecommunications systems may give rise to additional intercell interference considerations that should be addressed to optimise communications in such systems. For example, it is frequently proposed that virtual carriers may be particularly useful for supporting machine type communication devices and the inventors have recognised that such devices may frequently be located in areas of relatively poor network coverage, i.e. they may be “hard to reach” devices. For example, smart-meter type MTC devices might often be located in a basement or other location with relatively high penetration loss. This can mean high transmission powers may often be required to support reliable communications on virtual carriers, thereby giving rise to additional intercell interference concerns.
There is therefore a desire to provide wireless telecommunications apparatus and methods which are able to further help support communications with terminal devices with reduced intercell interference.